Recently I stumbled on a topic related to interconnecting 2 Juniper routers with Inter-AS Option B.

With this kind of connectivity it is not enough to have control plane working properly i.e. prefixes exchanged and visible in appropriate routing-instances on both ends – for data plane to work you need to have next-hop resolved – and it is implicit in case E-BGP peering is sourced with interface address. In case loopback is used as a source you need to have it resolved by:

either a LSP between the two ASBR (actually no label required, but it must appear in inet.3, it can even be a static dummy LSP)

or configure routing-options resolution rib bgp.l3vpn.0 resolution-ribs inet.0 which will allow L3VPN routes to be resolved in inet.0 instead of inet.3

More details can be found on the Juniper forum where I found the solution: